1. Field of the Invention
The present invention relates to an electret condenser microphone available for various audio equipments such as a cellular phone, and more particularly to an electret condenser microphone equipped with a capacitor unit constituted by an electrode plate and a diaphragm to receive an acoustic wave to be converted to an acoustic signal indicative of the acoustic wave.
2. Description of the Related Art
Up until now, there have been proposed a wide variety of conventional electret condenser microphones each equipped with a capacitor unit constituted by an electrode plate and a diaphragm to receive an acoustic wave to be converted to an acoustic signal indicative of the acoustic wave.
The conventional electret condenser microphones of this type have so far been available for various audio equipments such as a cellular phone. One typical example of the conventional electret condenser microphones is exemplified and shown in FIGS. 11 and 12. The conventional electret condenser microphone 900 thus proposed comprises a casing member 910 including a circular inlet portion 912 constituting an electrode plate, and a cylindrical side portion 913 integrally formed with the inlet portion 912 and having a cylindrical inner surface 913a. The side portion 913 of the casing member 910 has a first section 915 close to the inlet portion 912 of the casing member 910, and a second section 916 remote from the inlet portion 912 of the casing member 910 and radially inwardly bent.
The conventional electret condenser microphone 900 further comprises a covering member 981 provided on the inlet portion 912 of the casing member 910, and a printed circuit board 920 disposed in the casing member 910 to be held in contact with the second section 916 of the side portion 913 of the casing member 910. The printed circuit board 920 has first and second surfaces 920a and 920b each having thereon a printed wiring.
The conventional electret condenser microphone 900 further comprises an electrically connecting member 950 in the form of an annular ring shape and provided on the first surface 920a of the printed circuit board 920 to be disposed on and along the circumference of the printed circuit board 920. The electrically connecting member 950 has a cylindrical outer surface 950a smaller in diameter than the inner surface 913a of the side portion 913 of the casing member 910. The inner surface 913a of the side portion 913 of the casing member 910 has a cylindrical surface portion opposing the outer surface 950a of the electrically connecting member 950.
The conventional electret condenser microphone 900 further comprises a diaphragm 970 made of an electret film and mounted on the electrically connecting member 950. The diaphragm 970 includes a peripheral portion 971 fixedly supported by the electrically connecting member 950, and a central portion 972 integrally formed with the peripheral portion 971 and radially inwardly extending from the peripheral portion 971 to be partly oscillatable with respect to the casing member 910.
The electrically connecting member 950 intervenes between the printed circuit board 920 and the peripheral portion 971 of the diaphragm 970 to have the printed circuit board 920 and the peripheral portion 971 of the diaphragm 970 electrically connected with each other.
The inlet portion 912 of the casing member 910 is formed with a plurality of acoustic apertures 917, 918 and 919 to have the acoustic wave transmitted to the diaphragm 970 through the covering member 981 and each of the acoustic apertures 917, 918 and 919 of the inlet portion 912 of the casing member 910.
The conventional electret condenser microphone 900 further comprises an electrically insulating spacer 980 intervening between the inlet portion 912 of the casing member 910 and the diaphragm 970 to have the inlet portion 912 of the casing member 910 and the diaphragm 970 spaced apart from each other at a predetermined space distance.
The inlet portion 912 of the casing member 910, i.e., the electrode plate, and the diaphragm 970 collectively constitute a capacitor unit 902 to generate an electrical capacitance corresponding to the space distance between the inlet portion 912 of the casing member 910 and the central portion 972 of the diaphragm 970 under the state that the acoustic wave is transmitted to the diaphragm 970 to have the central portion 972 of the diaphragm 970 partly oscillated with respect to the casing member 910.
The conventional electret condenser microphone 900 further comprises a signal converting unit 990 designed to convert the electrical capacitance generated by the capacitor unit 902 to the acoustic signal indicative of the acoustic wave transmitted to the diaphragm 970. The signal converting unit 990 includes a field effect transistor 991. The signal converting unit 990 is provided on the first surface 920a of the printed circuit board 920 to be surrounded by the electrically connecting member 950 with a sufficiently large space distance between the printed circuit board 920 and the diaphragm 970. The signal converting unit 990 is electrically connected to the inlet portion 912 of the casing member 910 through the printed wiring of the second surface 920b of the printed circuit board 920 and the side portion 913 of the casing member 910, and to the diaphragm 970 through the printed wiring of the first surface 920a of the printed circuit board 920 and the electrically connecting member 950.
The conventional electret condenser microphone, however, encounters such a problem that the sensitivity to the acoustic wave is decreased, resulting from the fact that the side portion 913 of the casing member 910 and the electrically connecting member 950 collectively constitute an unwanted capacitor unit to generate a stray capacitance corresponding to the area of the surface portion, opposing the outer surface 950a of the electrically connecting member 950, of the inner surface 913a of the side portion 913 of the casing member 910.